1. Field of the Invention
The present invention relates to a method for preparing an improved porous polymer bead. More particularly, it relates to a method for preparing an improved porous polymer bead, which can introduce a number of functional groups per unit mass, has a high reaction rate and yield in the functionalization and application reaction, has stability under shock and constant high pressure, and has a low diffusional limitation on reaction rates.
2. Description of the Prior Art
Polymer bead is a polymer that serves as a support for some components of a reaction system, that is, it contains an appropriate functional group that can be bound to chemical groups to function as a reagent, catalyst and protecting function, etc. Such a polymer bead are widely used in chemistry, which include, for example, a polymeric reagent, ion-exchange resin, polymer supported phase transfer catalyst and polymer supported transition metal complex. The polymer supports may be linear polymers forming the molecular solution in a good solvent, and crosslinked polymers, i.e., beads insoluble macroscopically but swelling in a good solvent. A polymer bead is widely used as a polymer support because of a number of advantages from the insolubility thereof. Namely, when expensive catalytic active sites were supported on the polymer bead. Recovery and regeneration of expensive catalysts are facilitated, since the polymeric catalysts can be simply separated from the reaction mixture by filtration.
Somewhat, chemical functionalization of the polymer bead and continuous use as a polymeric catalyst after functionalizing, is determined by physical characteristics of the polymer bead. Accessibility of the reactants to the functional groups of the polymer supported catalyst depends on the swelling of polymer size and volume of the pore-resin specific catalytic reaction. These properties are determined by the degree of crosslinking of the polymer bead and synthetic condition for preparation. Polymeric beads may be generally classified into gel-type and macroreticular type according to its morphology.
Gel-type bead may be prepared by direct suspension-polymerization of the monomer with a vinyl group and crosslinking agent with two vinyl groups in the absence of a solvent. However, the polymer matrices open up, as good solvents are absorbed into it. Soft gel network was formed, upon contacting, with the good solvent. Thus the formed gelullar polymer bead has a considerable solvent volume. The gel-type polymer bead which contains crosslinking agent of less than 1% has high swelling in a good solvent but deteriorates easily because it possesses a low mechanical strength in general. While the gel-type polymer bead having the degree of crosslinking, more than 8% has a high degree of mechanical stability but the reactant is difficult to access to the functional groups in polymer bead by permeation due to the low swelling. Namely, when the polymer bead have a high degree of crosslinking, the reaction rate in the polymer is slow. Gel-type polymer bead with a 2% degree of crosslinking is widely used to provide an appropriate diffusion rate for the reactant and the sufficient mechanical strength, and is to be treated with ease. There is, for example, a copolymer bead prepared by suspension-polymerization of 98% styrene and 2% divinylbenzene.
A method for preparation of a porous polymer bead is similar to that of a gel-type resin except for the addition of the pore-forming agent. In the case that the monomers and polymers are dissolved completely in a pore-forming agent, a highly swelling network is formed after suspension-polymerization. When such a swollen polymer is dried in a vacuum oven to remove the solvent, a part of polymer is deteriorated, but many macropores were formed. The degree of crosslinking must be about 20% to have a suitable mechanical strength of pores. Macroreticular-type bead also prepared by polymerization using the pore-forming agent to act not only as a good solvent for the monomer, but also as a precipitator for the produced polymer. The macroreticular-type polymer bead obtained by the later method maintaining a whole shape and pore volume at removing the pore-forming agent and the size of the pore of the resin is larger than when using a good solvent for the polymer as pore-forming agent. This bead has a high content of crosslinking agent and large permanent pore volume, so that the reactant is facilitated to access to the reactive functional group. Such gel-type and porous bead are selected according to use, difference of application in catalyst, organic synthesis, separation of mixture, are as follows. Gel-type polymer bead must be swollen by a suitable solvent so that the reactant may diffuse into resin. But, gel-type resin is widely used because it has a high reactivity and yield per unit volume in functionalization and application reaction, and possesses high loading capacity under a good solvent. The swollen gel-type polymer bead is not sensitive to sudden shock while it was deteriorated easily under the constant high pressure. Also, there is a problem that when it is applied to packed column reactor because of the change of volume due to swelling in a good solvent. Meanwhile, a porous polymer bead having a strong macropore possesses low loading-capacity, low diffusional limitation as compared with a gel-type bead a small change of volume in the solvent. It has a stability under the constant high pressure, while it was deteriorated easily under sudden shock.
Under these conditions, the present inventors have made extensive studies to provide an improved porous polymer bead which may obviate and mitigate the above-mentioned drawbacks. As a result thereof, the present inventors found an improved porous polymer bead having the advantages of gel-type and porous bead to complete the present invention.